1. Field of the Invention
The present invention relates to liquid crystal display devices and, more particularly, to liquid crystal display devices having a suitable structure for displaying images with high luminance in a whole screen thereof by reducing temperature dispersion an illuminating light source thereof and preferably by evening temperature distribution of the illuminating light source.
2. Description of the Related Art
In a liquid crystal display device being utilized as displaying means for a personal computers or a monitor for the other purposes (e.g. a video monitor), an image being generated in a liquid crystal display panel is visualized by irradiating the liquid crystal display panel with illuminating light and by emitting the illuminating light being transmitted or reflected by the liquid crystal display panel from a display side thereof.
The liquid crystal display device of this sort utilizes a liquid crystal display panel comprising a pair of substrates being stuck on one another with a certain space therebetween and having pixel-selector electrodes being formed thereon, and a liquid crystal layer being interposed in the certain space. At one surface or both surfaces of the liquid crystal display panel either a polarizer plate (or, film) is formed thereon, or an optical retardation plate (or, film) and a polarizer plate (or, film) are stacked thereon. In the thus constructed liquid crystal display panel, an image is generated by modulating orientation states of liquid crystal molecules of the liquid crystal layer at selected pixel portions. As the image being generated in the liquid crystal display panel is still invisible itself, therefore the liquid crystal display device having the liquid crystal display panel is constructed to irradiate the liquid crystal display panel with extraneous light thereto, and to make the liquid crystal display panel transmit or reflect the incident light thereto, so as to visualize the generated image at a screen of the liquid crystal display device.
There are two kinds of light sources of the aforementioned illuminating light: one is “Transparent mode (Transparent type)” using a light source being disposed at a back side (opposite to the display side) of the liquid crystal display panel, i.e. backlight, and another is “Reflective mode (Reflective type)” utilizing extraneous light entering the viewing side (also called, the display side).
The backlight type which has an illuminating light source (an illumination system, a luminaire) being disposed at the back side of the liquid crystal display panel comes into wide use for the liquid crystal display device being provided for a notebook-sized personal computer and a display monitor (display screen). On the other hand, since a power source being mounted in a device, for example, PDA (Personal Digital Assistants: miniaturized portable data terminals) has a small capacity for itself, the liquid crystal display device coming into use for the such device does not have an active illuminating light source like the backlight, but utilizes ambient light being incident thereupon as illuminating light thereof. However, for enabling use of the liquid crystal display device of this sort in a faintly lit environment or an absolutely extraneous light-free environment, some products thereof having a auxiliary light source thereto appear on the market.
Most of the liquid crystal display devices utilizing the extraneous light belong to the reflective type. The reflective-type liquid crystal display device comprises a reflector layer on an inner surface of a substrate (lower substrate) thereof being opposite to another substrate (upper substrate) thereof being disposed at the display side (the viewing side) thereof. The reflector layer is formed by forming a layer of metal or the like on the lower substrate, and then applying a surface-treatment thereto for making a surface thereof mirror-like or specular. The extraneous light comes into the reflective-type liquid crystal display device through the upper substrate at the viewing side thereof, and then is reflected by the reflector layer thereof. The reflective-type liquid crystal display device visualizes an image being generated thereby by emitting the thus reflected light through the upper substrate thereof. Furthermore, another kind of the reflective-type liquid crystal display device which has a lower substrate thereof being formed of a transparent material and a specular reflector being provided at a backside thereof (or, a rear side thereof: behind the lower substrate from a viewing side thereof) is known as well.
If the auxiliary light source is employed for the reflective-type liquid crystal display device, the illuminating light source thereof is usually assembled by stacking a light guide member on a liquid crystal display panel thereof and disposing a linear lamp (e.g. a cylindrical light source) at one of edges of the light guide member so as to propagate light emitted from the linear lamp in the light guide member.
Moreover, in some instances of the reflective-type liquid crystal display device being employed for the PDA or the like, so-called touch panel for inputting data or the like onto a display surface thereof with a pen or a finger is provided therefor. The touch panel of this sort may be stacked on an upper surface of the liquid crystal display panel, or may be stacked on the light guide member composing the auxiliary light source when the auxiliary light source is equipped for the liquid crystal display panel.
FIG. 11 shows an disassembled squint view (an exploded view) for explaining one of exemplary configurations of the conventional transparent-type liquid crystal display device. The backlight is constitute with a light guide plate 5 and a fluorescent lamp 8 being disposed at a side-edge of the light guide plate and both ends of the fluorescent lamp are equipped with rubber bushes (lamp holders) 9 also being provided for holding electric power supplying leads 10. The liquid crystal display panel 3 and the light guide plate 5 are incorporated into an intermediate mold case 4, and the fluorescent lamp 8 is disposed at a light source housing 4A being formed within the intermediate mold case 4.
The driving circuit boards 2A, 2B are installed at the peripheries of the liquid crystal display device 3. A lamp reflector sheet 7 is installed along a circumference of the fluorescent lamp 8, and then both the lamp reflector sheet 7 and the fluorescent lamp 8 are fixed between a lower frame of metal 6 and an upper frame 1 by sandwiching them together with the liquid crystal display panel 3 between these frames. Needless to say, the upper frame 1 has a window for exposing the screen of the liquid crystal display panel 3.
An cold cathode fluorescent lamp is often used as the fluorescent lamp 8. The cold cathode fluorescent lamp comprises a glass tube of several millimeters in an inside diameter thereof which has a fluorescent film (layer) being coated on an inner surface thereof and contains mixed gas of Ne (neon) and Ar (argon) or the like and Hg (mercury) being sealed therein, and a pair of electrodes installed at ends thereof being opposite to one another.
When high voltage of several hundred volts (V) is applied between the pair of electrodes of the fluorescent lamp 8, electric discharge between both of the electrodes heats an inside thereof and generates mercuric vapor therein, and ultraviolet (UV) light being generated by excitation of the mercuric vapor stimulates the fluorescent film so that the fluorescent film emits visible light. Therefore, as the inside of the fluorescent lamp is heated higher by increasing currents applied thereto, a higher luminance is obtained thereby.
It is known that an electrooptical characteristic of the liquid crystal display panel 3 varies largely with ambient temperature thereof. Although the cold cathode fluorescent lamp is employed as the fluorescent lamp 8, heat quantity being generated at an electrode thereof is still so large that a temperature around the electrode rises up to 80° C. or higher when the current flowing thereto is 4 milliamperes (mA) in its effective value. Leaving the heat appearing around the electrode as it is, the heat is transmitted to the liquid crystal display panel 3, and deteriorates an image quality thereof by bleaching a part of a screen thereof.
For preventing such a deterioration of the image, the holder 9 shown in FIG. 11 is formed of silicone resin materials which has sufficient thermal conductivity and insulates impressed voltage of c.a. 1000 V being applied to the electrode of the cold cathode fluorescent lamp when it is turned on.
The lower frame 6 is made of aluminum (Al) for diffusing the heat throughout the display surface of the liquid crystal display panel 3 by its good thermal conductivity and for reducing gross weight thereof.
As mentioned above, each of the conventional lamp holder 9 and the conventional lower frame 6 has a function and a structure for dispersing the heat being emitted from the cold cathode fluorescent lamp.
Recently, a double-piped cold cathode fluorescent lamp has been proposed as a suitable way for suppressing the heat generated therefrom and moreover for obtaining high luminance thereof. The double-piped cold cathode fluorescent lamp is equipped with an additional transparent glass tube, covering a circumference of the conventional cold cathode fluorescent lamp and seals any appropriate decompressed gas in a space between the cold cathode fluorescent lamp and the additional glass tube, so as to keep the temperature of the cold cathode fluorescent lamp properly to obtain sufficient luminance thereof even if a current supplied thereto is relatively low. (see, Japanese Patent Application Laid-Open No. Hei 08-334760/JP-A-334760/1996)
By employing the cold cathode fluorescent lamp of the aforementioned double-piped structure as a light source of the liquid crystal display device, the liquid crystal display device with low power consumption becomes available.
Because miniaturized and lightweight portable data terminal devices are battery-powered, the liquid crystal display device (devices called PDA, or the like) being employed therefor is required especially to save power consumption. The portion of the liquid crystal display device which consumes electric power most is a luminaire section having a backlight (or, a front light) or the like. Therefore, the fluorescent lamp of the liquid crystal display device need to be operated by less tube currents as possible for saving power consumption thereof.
FIG. 12 shows a sectionally enlarged view for explaining a layout of structural elements in the backlight portion of the liquid crystal display device shown in FIG. 11. Each of the structural elements is drawn by solid lines for convenience.
In FIG. 12, by putting the lamp holder 9 installed at an end of the fluorescent lamp 8 in a fluorescent lamp retaining portion 4A provided by the intermediate mold frame 4, the fluorescent lamp 8 is retained at the certain position, i.e. the position in the vicinity of a side edge of the light guide plate 5. More specifically, an electric power supply lead (connector cable) 10 is connected to an electrode terminal 8A of the fluorescent lamp 8 by solder or the like, and the holder 9 serves as both a protector and an insulator for the connecting portion of the electric power supply lead 10 and the electrode terminal 8A.
The lamp reflector sheet 7 is provided around the fluorescent lamp 8 and the light guide plate 5 other than an area between the fluorescent lamp 8 and the light guide plate 5. Each edge of the lamp reflector sheet 7 in a longitudinal direction thereof is fixed on both the front and the back of the light guide plate 5 by fixing means suitable therefor, so that light emitted from the fluorescent lamp 8 is reflected effectively towards the light guide plate 5.
The lower frame 6 is disposed under the intermediate mold frame 4 having the fluorescent lamp retaining portion 4A. The lower frame 6 is formed of light metal, preferably of Aluminum, and functions to radiate heat from the fluorescent lamp.